The present invention relates to a synchronous motor which is controlled by a thyristor converter, and more particularly to a control apparatus employing a thyristor for controlling commutatorless motors.
In recent years, thyristor motors or commutatorless motors have been widely employed for driving heavy loads such as rolling mills. In the rolling mills, for instance, the driving motors are often required to produce large torque even while they are rotating at speeds near zero or at very slow speeds, and are further required to be continuously controlled. Under such conditions, however, since an alternating current supplied to an armature is zero or near zero in frequency and is scarcely commuted over the thyristors alternately, the electric current tends to be concentrated on any one of the thyristors of the converter. This phenomenon is called "current concentration", and is described in detail in U.S. Pat. No. 4,060,753 which is issued on Nov. 29, 1977 and assigned to Hitachi, Ltd. This phenomenon causes the junction portion of the thyristor to generate heat so that the junction portion is finally broken down. To cope with this current concentration, therefore, all of the thyristor circuits must have .pi. times of capacity. Because of this fact, it is necessary to employ a thyristor converter or a cycloconverter circuit having large capacity.
In order to solve the abovementioned drawbacks, a control system has been proposed in the U.S. Pat. No. 4,060,753, in which two field windings are provided perpendicularly intersecting to each other, and not only the armature windings but also the field windings are rotationally excited by an alternating current. If the angular frequency of the field excitation current produced by output signals S.sub.1 and S.sub.2 of an oscillator is denoted by .omega..sub.S, the terminal voltage and armature current will have the angular frequency .omega..sub.s +.omega..sub.r, in which .omega..sub.r represents the angular frequency of the rotational speed of the motor. In such a system, when the rotational speed is zero (.omega..sub.r =0), the armature current has the frequency .omega..sub.s. Therefore, the current concentration can be prevented.
However, the abovementioned system presents the following drawback when it is applied to the motor which is designed to rotate in both directions. Namely, when the motor is required to rotate at a speed -.omega..sub.s, i.e., at a speed .omega..sub.s in the reverse direction, the frequency of the armature current becomes zero. Therefore, the current concentration occurs in the converter circuit. Otherwise, the current concentration occurs when the motor rotates at the speed .omega..sub.s with the armature current being controlled with the angular frequency .omega..sub.r -.omega..sub.s.